Use of non-stick coatings is well known in the art. The prior art discloses a myriad of non-stick coatings and applications for such coatings. The composition of non-stick coatings varies and each variation brings different considerations, many of which are not particularly useful in the field of welding applications. As discussed below, innovations for improved non-stick coatings are driven by considerations related to developing coatings that can be conveniently applied, do not otherwise interfere with the operability or functionality of the underlying device or structure, include environmental considerations, and/or are more effective at prolonging the operating life of underlying devices through an increased resistance to degradation of the coating due to exposure to harsh operating environments—such as welding or metal working environments.
One exemplary device commonly exposed to a welding environment are proximity or location sensors associated with confirming the location of parts intended to be welded together. Such sensors are frequently electromagnetic in nature and multiple such sensors are commonly employed in various industries such as in automobile assembly lines to detect the position of parts during welding operations that are commonly conducted by robotic welders. Such sensors are often positioned in close proximity to the welding operation such that the sensors can be directly exposed to a spatter of hot material and/or slag associated with such metal working processes. Unless the slag falls off the sensor, the slag accumulates on the sensor and, if left unaddressed, can render the sensor inoperative. Most assembly lines include multiple such proximity sensors and the downtime that results from the delay needed to replace any of the sensors increases per unit production costs from both the delay of production operations as well as the time and personnel investment associated with the physical repair and/or replacement of inoperable sensors.
In assessing the ability of a surface to withstand such an interaction, various parameters of the surface are assessed. Erosion wear due to impinging particles on a target material surface can be modeled as:
      V    =                  dE                  5          4                                      H                      17            12                          ⁢                  K          IC                      ;where V is the volume of the target surface eroded, d is the density of the material, E is the modulus of elasticity or elastic modulus, H is the material hardness, and KIC is the fracture toughness of the target or afflicted surface. In the case of proximity sensor operation, greater erosion wear of the sensor face leads to increases in the density or concentration of pits and craters formed in the sensor face. Such pits or craters provide surface discontinuities that expedite the accumulation of weld slag or spatter on the sensor. The exposed sensor can fail if the slag accumulated on a particular sensor exceeds a threshold value and such sensors commonly fail to a part present signal. That is, the sensor can be said to be locked-on in that the buildup of metallic slag on the sensor is improperly interpreted by the sensor as a part always being present at a desired location.
In resolving the surface degradation discussed above, surface hardness has been shown to have a greater effect on reducing erosion wear than the fracture toughness of the afflicted material. Increasing the hardness value associated with a surface or coating has been a popular approach to mitigate weld slag accumulation. Unfortunately, such an approach provides only limited benefit in mitigating undesired slag accumulation.
In addition to mitigating surface degradation, it is also known that the force of stiction, or the frictional force that must be overcome to allow motion between stationary contacting objects—such as slag already undesirably adhered to a sensor or structure, can be manipulated by different surface materials or treatments. Stiction is related to the elastic modulus and surface energy of a material and can be modeled as:
      F    =          γ      E        ;where γ is surface energy and E is the elastic modulus. Therefore, reducing the value of the surface energy will reduce the value of the stiction force associated with the surface and thereby reduce the ability of slag to collect on the particular surface.
Fluoropolymer based coatings, such as polytetrafluoroethylene (or PTFE) coatings—such as Teflon® (a registered trademark of DuPont), can be used to reduce the weld slag adhesion performance of exposed sensors, as they have low surface energy. However, low surface energy polymers such as PTFE do not have high hardness and, over time, weld slag particles abrade the PTFE coating such that discontinuities or pits develop in the exposed surface of the coating. The pitting or surface porosity provides the initial surface discontinuity that promotes weld slag adhesion. Over time, weld slag accumulates on the sensor thereby degrading operability of the sensor and ultimately, if left unaddressed, renders the sensor inoperable so as to require replacement of the sensor to maintain process operations.
Various approaches have been undertaken to reduce the detrimental effects of slag collection on such sensors. U.S. Pat. No. 4,996,408 by Turck, et al., entitled “Proximity switch for use in welding facilities,” discloses a polytetrafluoroethylene (or PTFE) with perfluoroalkoxy side chains based non-stick coating that can be applied to proximity switches. The reference discloses mixing coloring pigments with the PTFE material however the coloring pigments do not otherwise manipulate the anti-adhesion properties of the coating. Many such currently available coatings also tend to be non-aqueous based such that, manufacturing and use of such coating materials tends to be expensive as such manufactures must also satisfy various volatile organic carbon (VOC) emission manufacturing standards.
Still others have attempted to manipulate the slag adhesion of fluoropolymer coatings through the use of fillers to increase the hardness and reduce the surface energy of the fluoropolymers. U.S. Pat. Nos. 7,968,640; 8,202,930; and 8,207,257 to Ganguli et. al. describe various composite coatings with acidified graphite and fluoropolymers that provide improved nonstick performance over other asserted nonstick coatings. The acidified graphite increases hardness while retaining comparatively low surface energy. Additionally, the graphite increases the thermal conductivity of the coating thereby reducing local heat generation and surface pitting attributable to thermal decomposition of the polymer of the coating.
Other solutions to the problem of undesired weld slag adhesion and collection include alternate construction materials and/or specialized protective accessories. U.S. Pat. No. 6,617,845 to Shafiyan-Rad, et al., entitled “Proximity sensor resistant to environmental effects,” discloses a housing configured to encase a sensor. The housing is disclosed as being made of a material with high thermal conductivity, such as copper or an alloy thereof. Such a device uses the inherent properties of the material of a housing extraneous to the sensor to disperse the heat associated with slag adhesion but does not provide a coating that can be economically applied to other structures or devices intended to be protected.
Each of the methodologies discussed above provide coatings or shield structures that are intended to protect an underlying structure but each of the methodologies discussed above also provide only opaque structures or coatings. When such coating methodologies are used, application of an opaque coating, while improving the slag adhesion performance, negates the use of any markings and/or indicators such as LED's that may be supported by the underlying structure, such as a proximity sensor. The opaque nature of such coatings also limits the types of sensors that can be coated and prevents use of the coating on other transparent weld accessories, such as inspection shields, goggles, and/or facemasks.
Therefore, there is a need for a coating that provides improved anti-stick weld slag performance and which protects the longevity associated with the underlying device. There is also a need for a coating that does not interfere with inspection of indicia associated with the underlying device and/or which is applicable or usable on other non-opaque devices or structures that may be exposed to the welding environment.